networks: Remove firewall zone warning in network connection information

When an interface is not assigned a firewall zone, we currently show a message as follows:

This interface is not maintained by FreedomBox. Its security status is unknown to FreedomBox. Many FreedomBox services may not be available on this interface. It is recommended that you deactivate or delete this connection and re-configure it.

This message is incorrect. All interfaces that are not assigned a zone now automatically in the external zone. We need to remove the warning and show that the firewall zone is external due to automatic assignement.